An example of an electrical load which is directly connected to a supply susceptible to power transients, is a mains-driven LED circuit.
Presently the majority of mains powered LED lamps use DC/DC converters in order to adapt the high mains voltage to the low LED voltage. However, this approach is not particularly convenient, involves additional components, and is relatively complex to design, especially when applied as a retrofit to existing lamps and lamp fixtures. As a result there is an increasing interest in modifying the LED load such that it is capable of operating directly from a mains voltage. One such variable LED load is disclosed in patent application publication W02008-060469.
However, if the LED load is suitable for operating directly from the mains, then the LED drive electronics should also be driven directly from the mains supply.
A problem then arises: typical, and even high-end, integrated circuits based on silicon have a maximum allowed voltage of about 800V. In contrast, the surges on the mains can often be in the 1 to 2 kV range on top of the mains AC voltage of perhaps ±400V. The level of transients arising as a result of surges which can be expected on a mains supply are described for example in the International Electrotechnical Commission's document IEC61000-4 on Standard of Transient Immunity Requirements. Such transients arise typically when there is a sudden change in the overall load supplied by the mains supply. For instance, when an inductive load such as a conventional motor is switched on, there can be a large, short-term, back-EMF generated, which appears on the mains as a spike or transient. A similar situation arises when an inductive load is interrupted.
During a mains surge, the voltage rises to the 1 to 2 kV region within a few microseconds, after which it decays within a few tens of microseconds. Surge protection circuits are known, which typically involve an energy absorber such as a varistor or a capacitor, often combined with a resistor. Such systems are bulky and introduce additional components into the circuit, as well as increasing the costs of the circuits, and are thus undesirable.
There thus an ongoing requirement for a cost-effective solution to the problem of surge protection for electrical loads which are directly connected to a supply which is susceptible to transients.
Although the problem has been described in relation to LED loads which are directly connected to a mains supply, it will be readily appreciated that a directly analogous problem exists with other loads which are directly connected to a voltage supply, which incorporate control electronics or drive electronics which may not be compatible with a transient surge in the supply. Examples of such systems include mains voltage fan supplies with additional functionality such as the fan speed control which is dependent on temperature, relay controls running from either mains voltage or a typical control voltage of 24V, or mains timers.
Further, it will be appreciated that the LED loads to which this invention applies are not limited to crystalline semiconductor-based LEDs, but embrace LEDs based on alternative technologies, such as but not limited to OLEDs (organic LEDs), POLYLEDs (Polymer LEDs), smOLEDS (small-molecule and the like